WEST DES MOINES — A new billboard in Des Moines and comments from a Republican colleague have rekindled the debate, but Chuck Grassley remains adamant that the U.S. Senate should not hold hearings on the president’s U.S. Supreme Court nominee until next year, after a new president and Congress are sworn in.

Grassley, the Iowa Republican who leads the Senate Judiciary Committee through which Supreme Court nominations must pass, has joined GOP Senate leadership in blocking President Barack Obama’s Supreme Court nominee, saying the vacancy — created in February by the death of Justice Antonin Scalia — should not be filled during an election year.

For that, Grassley has drawn criticism and political pressure from Democrats and liberal advocacy groups.

One such group, Progress Iowa, unveiled a new billboard advertisement Tuesday near downtown Des Moines that highlights the issue.

And on Monday, U.S. Sen. John McCain said in a radio interview that Senate Republicans will be “united against any Supreme Court nominee” presented by Democratic presidential candidate Hillary Clinton, should she win.

McCain’s office later said the senator will examine any Supreme Court nominee and vote for or against the individual, and that his comment was meant to reflect his belief that Clinton “has a clear record of supporting liberal judicial nominees.”

“They’re basically saying that we’re going to process nominees, but whether we vote for them or not is a completely different thing,” Grassley told reporters Tuesday after holding a public event at UnityPoint Health-Methodist West Hospital. “I think based on the letter I signed on Feb. 23 to my colleagues, that we ought to let the people have a voice and a new president make a decision. It means that we will advise hopefully before they name somebody.”

Grassley is running for re-election to the U.S. Senate. His Democratic challenger is Patty Judge, a former Iowa agriculture secretary and lieutenant governor.

Grassley said he hopes the next president, whether it is Clinton or Republican candidate Donald Trump, nominates a “dispassionate” judge unlike current Justices Stephen Breyer, Ruth Bader Ginsburg, Elena Kagan and Sonia Sotomayor. Grassley said they “vote as a bloc all the time in an ideological way and aren’t always dispassionate.”

Grassley said he approves of a list of potential nominees presented by Trump, saying the judges “have been vetted by a lot of organizations that believe the Supreme Court ought to interpret law and not make law.”

The new Progress Iowa billboard, which is part of a $20,000 advertising campaign, according to the organization’s executive director, features a photo of Grassley and text that reads, “Obstruction is not an Iowa value” and points observers to the website chuckobstructs.com.

West Des Moines resident Dan Towers, who attended a Progress Iowa news conference to unveil the billboard ad, said Tuesday that as a U.S. Army veteran, he took the same oath to uphold the U.S. Constitution as do U.S. senators. He implored Grassley to honor that oath by holding a hearing on Obama’s Supreme Court nominee.

“I’ve voted for Sen. Grassley in the past. I’ve admired Sen. Grassley’s integrity, the fact that he always represented Iowans with a voice of principle and integrity,” Towers said. “I’m asking Sen. Grassley and the senators to not only do your job in holding hearings, but to uphold your sworn duty to uphold the Constitution of the United States.”

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